The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to methods and apparatus for measuring the thickness of a substrate layer during chemical mechanical polishing.
An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the carrier head configuration, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
In order to determine the effectiveness of different polishing tools and processes, a so-called xe2x80x9cblankxe2x80x9d wafer, i.e., a wafer with multiple layers but no pattern, is polished in a tool/process qualification step. After polishing, the remaining layer thickness is measured at several points on the substrate surface. The variation in layer thickness provide a measure of the wafer surface uniformity, and a measure of the relative polishing rates in different regions of the substrate. One approach to determining the substrate layer thickness and polishing uniformity is to remove the substrate from the polishing apparatus and examine it. For example, the substrate may be transferred to a metrology station where the thickness of the substrate layer is measured, e.g., with an ellipsometer. Unfortunately, this process can be time-consuming and thus costly, and the metrology equipment is costly.
One problem in CMP is determining whether the polishing process is complete, i.e., whether a substrate layer has been planarized to a desired flatness or thickness. Variations in the initial thickness of the substrate layer, the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate can cause variations in the material removal rate. These variations cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot be determined merely as a function of polishing time.
One way to determine the polishing endpoint is to remove the substrate from the polishing surface and examine it. For example, the substrate may be transferred to a metrology station where the thickness of a substrate layer is measured, e.g., with an ellipsometer. If the desired specifications are not met, the substrate is reloaded into the CMP apparatus for further processing. This is a time consuming procedure that reduces the throughput of the CMP apparatus. Alternatively, the examination might reveal that an excessive amount of material has been removed, rendering the substrate unusable.
There is, therefore, a need for a method of measuring in situ the thickness and flatness of the substrate layer, and detecting whether the desired thickness or flatness has been achieved.
Several methods have been developed for in-situ polishing endpoint detection. Most of these methods involve monitoring a parameter associated with the substrate surface, and indicating an endpoint when the parameter abruptly changes. For example, where an insulative or dielectric layer is being polished to expose an underlying metal layer, the coefficient of friction and the reflectivity of the substrate will change abruptly when the metal layer is exposed.
Where the monitored parameter changes abruptly at the polishing endpoint, such endpoint detection methods are acceptable. However, as the substrate is being polished, the polishing pad condition and the slurry composition at the pad-substrate interface may change. Such changes may mask the exposure of an underlying layer, or they may imitate an endpoint condition. Additionally, such endpoint detection methods will not work if only planarization is being performed, if the underlying layer is to be over-polished, or if the underlying layer and the overlying layer have similar physical properties.
In general, in one aspect, the invention features a method of measuring a characteristic of a layer on a substrate during chemical mechanical polishing. A surface of the substrate is brought into contact with a polishing pad that has a window. Relative motion is created between the substrate and the polishing pad. A light beam is divided through the window, and the motion of the polishing pad relative to the substrate causes the light beam to move in a path across the substrate surface. An interference signal produced by the light beam reflecting off the substrate is monitored, and a plurality of intensity measurements are extracted from the interference signal. Each intensity measurement corresponds to a sampling zone in the path across the substrate surface. A radial position is determined for each sampling zone, and the intensity measurements are divided into a plurality of radial ranges according to the radial positions. The characteristic is computed for each radial range from the intensity measurements associated with that radial range.
Implementation of the invention may include one or more of the following features. The characteristic may be a polishing rate, an initial thickness of the substrate layer, a remaining thickness, or a difference between the initial thickness and the remaining thickness of the substrate layer. A measure of polishing uniformity may be calculated from the measured characteristic in each radial range. A model function, such as a sinusoidal function, may be determined for each radial range. The sinusoidal function may be described by a period and a phase offset, in which may be computed from a least square fit of the model function to the intensity measurements in the associated radial range. The intensity measurements may be extracted by integrating the interference signal over a series of sampling times. Each sampling zone may correspond to a portion of the substrate across which the light beam travels during a corresponding sampling time. A time when the window crosses a midline of the substrate may be determined, and a position of the polishing pad may be determined from a difference between a measurement time and the time when the window crosses the midline of the substrate. The substrate may be positioned on the polishing pad by a carrier head having a retaining ring, and the time that the window crosses the midline of the substrate and may be determined from a first time and a second time when the window passes beneath the retaining ring. Determining the time the window crosses the midline of the substrate may be determined from a signal from a position sensor which monitors the position of the polishing pad. The radial position may be determined from a head sweep profile. Intensity measurements from sampling zones having radial positions greater than a predetermined radius may be discarded. The polishing pad may be located on a platen, and the platen may be rotated to create the relative motion between the substrate and the polishing pad. The light source, e.g., a laser, may be connected to and may rotate with the platen.
In another aspect, the invention is directed to a method of measuring a characteristic of a layer on a substrate during chemical mechanical polishing. A surface of the substrate is contacted with a polishing pad, and a light beam is directed through a window in the polishing pad onto the substrate. The light beam moves in a path across the substrate surface. A plurality of intensity measurements are produced by the reflection of the light beam from the substrate. The intensity measurements are divided into a plurality of radial zones according to the radial position of the light beam on the substrate during the intensity measurement, and the characteristic is computed for each radial zone from the intensity measurements associated with that radial zone.
In another aspect, the invention is directed to a chemical mechanical polishing apparatus. The apparatus includes a movable polishing surface that has a window and, a carrier head for holding a substrate having a layer thereon in contact with the polishing pad. A light source directs a light beam through the window, and the motion of the polishing pad relative to the substrate causes the light beam to move in a path across the substrate surface. A detector monitors an interference signal produced by the light beam reflecting off the substrate. A computer is configured to extract a plurality of intensity measurements from the interference signal, determine a radial position for a sampling zone corresponding to each intensity measurement, divide the intensity measurements into a plurality of radial ranges according to the radial positions, and compute a characteristic of the substrate layer for each radial range from the intensity measurements associated with that radial range.
Implementation of the invention may include the following. The carrier head may include a retaining ring with a reflective lower surface. A position sensor may monitor the position of the polishing pad and the carrier head.
Advantages of the invention may include one or more of the following. The thickness of a substrate layer on a blank wafer may be measured in-situ at a plurality of radial positions in order to generate a measure of the polishing uniformity to characterize the effectiveness of the CMP tool and process. The thickness measurements may be used to determine the endpoint criteria or to adjust polishing parameters to improve polishing uniformity. The thickness measurements may also be performed when polishing a device wafer to detect the polishing endpoint.
Other features and advantages of the invention will become apparent from the following description, including the drawings and claims.